In recent years, a non-volatile storage device (MRAM: Magnetoresistive Random Access Memory) using a magnetic memory element has been developed. For example, a magnetic memory element shown in FIG. 1 is given as an example of the related art. FIG. 1 is an enlarged cross-sectional view illustrating a portion including a magnetic memory element 100 in a storage device 10 including the magnetic memory element 100. The magnetic memory element 100 includes a magnetic tunnel junction (MTJ) portion 13, and the MTJ portion 13 is sandwiched between a lower electrode 14 and an upper electrode 12. The MTJ portion 13 has a multi-layered structure of a fixed layer 22, an insulating layer 21, and a recording layer 20 formed on the lower electrode 14 in this order. The fixed layer 22 and the recording layer 20 are made of a ferromagnetic material. The lower electrode 14 is connected to a drain region 24 provided in a substrate 15, and a source region 25 is also provided in the substrate 15 at a predetermined distance from the drain region 24. A gate line 16 is formed above the drain region 24 and the source region 25 so as to be insulated therefrom. In this way, a MOSFET (metal oxide semiconductor field effect transistor) having the terminals of the drain region 24, the source region 25, and the gate line 16 is formed. In addition, a contact portion 17 and a word line 18 are formed in this order on the source region 25. The upper electrode 12 is connected to a bit line 11. The word line 18 and the bit line 11 are insulated from each other by an interlayer insulating film 23 and are connected to a control circuit (not shown). The storage device 10 selects the magnetic memory element 100, reads information stored in the magnetic memory element 100, and writes information to the magnetic memory element 100.
Next, the principle of a data read operation of the magnetic memory element 100 will be described. First, the insulating layer 21 is provided between the recording layer 20 and the fixed layer 22, and the insulating layer 21 has a small thickness of 3 nm or less. Therefore, when an external voltage is applied, a small amount of current (tunnel current) flows from the recording layer 20 to the fixed layer 22 through the insulating layer 21. Since the recording layer 20 and the fixed layer 22 are ferromagnetic bodies, they have spontaneous magnetization (hereinafter, simply referred to as “magnetization”), and the tunnel current is increased or decreased by a combination (magnetization arrangement) of the magnetization directions of the recording layer 20 and the fixed layer 22. That is, when the direction of the magnetization 102A of the recording layer 20 is identical to (parallel to) the direction of the magnetization 102B of the fixed layer 22, the tunnel current passing through the insulating layer 21 increases. On the other hand, when the direction of the magnetization 102A of the recording layer 20 is opposite to (anti-parallel to) the direction of the magnetization 102B of the fixed layer 22, the tunnel current decreases. This property is called a tunneling magnetoresistance effect, which is described in detail in Non-patent Document 1: Inomata Koichiro, “Non-volatile magnetic memory MRAM,” Kogyo Chosakai Publishing Co., Ltd., November 2005 (hereinafter Non-patent Document 1).
This property can be used to determine whether the magnetization directions of the recording layer 20 and the fixed layer 22 are identical to each other, which is defined as “0”, or the magnetization directions of the recording layer 20 and the fixed layer 22 are opposite to each other, which is defined as “1”, on the basis of the magnitude of the tunnel current. That is, when the direction of the magnetization 102B of the fixed layer 20 is fixed, it is possible to read information stored in the recording layer 20 as the magnetization direction. The magnetization directions of the recording layer 20 and the fixed layer 22 are maintained even when energy, such as a current, is not supplied. Therefore, when the magnetic memory element 100 shown in FIG. 1 is integrated, it is possible to achieve a non-volatile storage device (memory) that retains data even when the power source is turned off.
Next, the principle of a data write operation will be described. In the related art, in order to write data, a method has been used in which a magnetic field is generated in the vicinity of the recording layer 20 due to a current and the magnetization direction of the recording layer 20 is changed by the magnetic field. However, in this method, as the size of an element is reduced, the amount of current required for a write magnetic field increases. As such, since the current value increases with a reduction in the size of the element, it is difficult to reduce the size of the magnetic memory element, that is, to increase recording density. Therefore, in recent years, a method has been used which makes a spin-polarized current flow from the fixed layer 22 to the recording layer 20 to control the magnetization direction of the recording layer 20. This method is called an STT (Spin Torque Transfer) method and is described in detail in Non-patent Document 1. In the STT method, a spin-polarized current for writing is reduced with a reduction in the size of the element. Therefore, it is easy to increase recording density.
A gigabit-class magnetic memory device has been developed by the use of the perpendicular magnetization film and the STT method. The magnetic memory element shown in FIG. 1 has the same operation as the magnetic memory element disclosed in Patent Document 1: U.S. Patent Application Publication No. 2007/297220.